The invention relates to novel boron, phosphorus or boron-aluminium dopant pastes for the production of p, p+ and n, n+ regions in monocrystalline and polycrystalline Si wafers, and of corresponding pastes for use as masking pastes in semiconductor fabrication, power electronics or in photovoltaic applications.
Boron or phosphorus doping of monocrystalline Si wafers in photovoltaic technology and in the case of power semiconductors (e.g. diodes, thyristors) is carried out via the vapour phase (e.g. POCl3, phosphine, BBr3) or with solids (films or solid slices) or with liquid media such as, e.g. phosphoric acid, and other organic B or P systems.
Directly patterned application is not possible with these techniques. Defined regions cannot be selectively doped without using elaborate, e.g. photolithographic processes, to mask the areas that are not to be doped.
Directly patterned application is not possible with these techniques. Defined regions cannot be selectively doped without using elaborate e.g. photolithographic processes to mask the areas that are not to be doped.
In photovoltaic technology, an extensive or local back surface field (BSF) can be achieved by p++ doping on the back. Existing production lines currently operate only with an extensive back surface field, which is obtained by an Al metal layer. Local BSFs have currently only been realized on a laboratory scale, for reasons which will be explained in more detail below.
An extensive back surface field is especially advantageous, inter alia, in the case of thin Si wafers (xe2x89xa6250 xcexcm), since otherwise the efficiency of the cell turns out to be significantly lower. Such a BSF is currently obtained in the case of Si wafers of  greater than 300 xcexcm thickness by extensive printing with an aluminium paste. This case is advantageous in that, on the one hand, p+ doping, i.e. a BSF, is built up by the aluminium and, on the other hand, the electricity generated in the cell is also dissipated by the aluminium. In the case of thin wafers, however, curvature of the wafer occurs after the aluminium paste has been burned in. The further handling and processing of such wafers therefore requires special measures since there are pronounced difficulties involved with the incorporation of such bent cells into the usually planar-designed PV modules.
In DE 195 08712 C2 and in K. A. Mxc3xcnzer, R. R. King, R. E. Schlosser, H. J. Schmidt, J. Schmalzbauer, S. Sterk, H. L. Mayer, 13th European Photovoltaic Solar Energy Conference, 23-27 10.1995, p. 1398, attempts are made to solve this problem by a spin-on process with boron. In this case, p++ doping is produced by boron and an aluminium framework is implemented in a subsequent print by means of aluminium paste.
Disadvantages of this process are
a) the large material requirement of the spin-on process
b) the considerable equipment outlay, with which angular wafers are to be coated uniformly using the spin-on process
c) the high throughput and the expensive handling, which are to be realized in mass production only with great difficulty, and
d) that selective patterning of the wafers is not possible, as described above.
The object of the present invention was therefore to provide improved, inexpensive dopant pastes usable in semiconductor technology, which do not have the disadvantages described above and can be used in a straightforward way. The object of the present invention was also to provide corresponding screen-printable pastes.
The object is achieved by dopant pastes for the selective patterning and for the extensive printing of Si wafers for the production of p, p+, p++, n, n+, n++ regions in the Si wafer, containing
a) one or more components with dopant action,
b) an SiO2 matrix,
c) solvents,
d) a thickening agent or wetting agent,
e) optionally an acid and water, and optionally
f) additives,
the total composition having impurities in the form of metal ions in respective concentrations of less than 200 ppb, preferably less than 100 ppb.
Through tests, it was found that the disadvantages mentioned above are eliminated by the screen-printable dopant pastes found here.
The pastes according to the invention may contain a dopant source selected from the group boron salt, boron oxide, organic boron compounds, boron-aluminium compounds and phosphorus salt, phosphorus oxide, phosphorus pentoxide, phosphoric acid, organophosphorus compounds and organic aluminium compounds.
In principle, the compounds known to the person skilled in the art primarily from main groups III and V of the periodic table are suitable.
The corresponding pastes furthermore contain an SiO2, matrix consisting of one or more SiO2 precursors.
The dopant pastes which have been found may contain one or more dopant sources selected from the group boron oxide (B2O3), phosphoric acid (H3PO4), phosphorus pentoxide (P2O5) and aluminium salt, it being possible for the doping sources to be used in different concentrations.
The present invention also relates to dopant pastes which contain one or more dopant sources from the group of organic boron, phosphorus and aluminium compounds, it being possible for the doping sources to be used in different concentrations.
The object is furthermore achieved by dopant pastes, containing one or more dopant sources selected from the group boron oxide (B2O3), phosphoric acid (H3PO4), phosphorus pentoxide (P2O5) and aluminium salt, and optionally one or more dopant sources from the group of organic boron, phosphorus and aluminium compounds, it being possible for the doping sources to be used in different concentrations.
The SiO2 matrix, which the dopant paste contains, may according to the invention be formed by an organic silicon compound of the general formula Rxe2x80x2nSi(OR)4xe2x88x92n, in which
Rxe2x80x2 denotes methyl, ethyl or phenyl,
R denotes methyl, ethyl, n-propyl or i-propyl, and
n denotes 0, 1 or 2.
The present invention furthermore relates to a dopant paste that contains an SiO2 matrix, which is formed by silanes, selected from the group tetramethoxysilane, tetraethoxysilane, tetrabutoxysilane, and tetrapropoxysilane, individually or as a mixture.
The present invention also relates to the use of the described new dopant pastes in semiconductor technology, photovoltaic technology or in power electronics.
Besides the dopant pastes, the invention also relates to masking pastes, containing
a) an SiO2 matrix,
b) solvents,
c) a thickening agent or wetting agent,
d) optionally an acid and water, and optionally
e) additives,
the total composition having impurities in the form of metal ions in respective concentrations of less than 200 ppb, preferably less than 100 ppb.
The masking pastes, like the dopant pastes mentioned above composed in the same way, except that the masking pastes are free from dopants, are used according to the invention in semiconductor technology, photovoltaic technology and in power electronics.
It is likewise possible to achieve extensive or selective (selective emitter) phosphorus doping inexpensively and with high throughput by means of screen-printable pastes.
Patterning stages, which are currently made possible by photolithographic processes, can be replaced inexpensively by the use of screen-printable dopant paste.
The masking paste, which can be described as pure SiO2 matrix paste without dopant additives, can be employed for intentionally defined protective layer formation. For this purpose, the paste my be applied extensively to the Si wafer, or else in patterned fashion. This makes it possible to protect defined regions from doping during the diffusion process.
The object is achieved by novel pastes for the selective production of silicate layers doped with phosphorus, boron and boron-aluminium. These pastes are suitable for use in technical printing processes in the electronics industry. These include, especially, screen-printing technology or pad printing.
Depending on the desired field of application, both the concentrations of the individual components and the viscosities may be varied in the pastes that are used.
The dopant pastes according to the invention are furthermore distinguished by a defined matrix. This is formed by an SiO2-forming component during heating in the diffusion process. Together with the component delivering boron or phosphorus, what is described in the literature and known to the person skilled in the art as boro- or phosphorus silicate glass (B2O3)x*(SiO2)y or (P2O5)x*(SiO2)y is formed. This glass forms the actual source of the dopant during the doping.
The quantity of the SiO2-forming component in the dopant paste is in a modifiable range of 0.1-5% by weight. In relation to the dopant components B2O3 or P2O5, the ratio of SiO2 to B2O3 or P2O5 may be set arbitrarily, although the range of 10-80% dopant in SiO2 is advantageous. The dopant concentration to be achieved in the silicon can be controlled well through this ratio. The penetration depth of the dopant is, as known in the literature, controlled through the parameters of diffusion time and diffusion temperature.
The doping process is carried out by controlled diffusion in air or oxygen atmosphere. The temperatures for the phosphorus diffusion are, e.g. 900-950xc2x0 C., and for the boron diffusion about 950-1050xc2x0 C. The diffusion time may, depending on the application purpose, be from a few minutes to as much as 60 hours or more. Following the diffusion, the wafers are treated with HF (concentration about 2-50% HF), in order to dissolve the phosphorous or borosilicate (PSG, BSG) glass formed. After this treatment, the Si wafer has a free Si surface.
The dopant component is in the concentration range of 0-10% in relation to the dopant paste. In the case of a dopant-free (0% by weight) composition, the masking paste is produced. This is done under the same process conditions as in the case of the dopant pastes.
The solvent forms the main constituent of the paste. The proportion may be in the range of 50-80% by weight of the total quantity.
The proportion of the thickening agents, which are needed for controlled adjustment of the viscosity range and fundamentally for the printability of the dopant, i.e. to form a printable paste, is in the range of about 1-20% by weight in relation to the total quantity of the paste.
Other additives with properties advantageous for the desired purpose are antifoaming agents, adhesion moderators, levelling agents or thixotropic agents. These can positively affect the printability of the paste.
Alkoxysilanes are generally suitable for forming the silicon matrix, in particular silanes of the general formula Rxe2x80x2nSi(OR)4xe2x88x92n,
in which
Rxe2x80x2 denotes methyl, ethyl or phenyl,
R denotes methyl, ethyl, n-propyl or i-propyl, and
n denotes 0, 1 or 2.
Examples of suitable silanes are tetramethoxysilane, tetraethoxysilane, tetrabutoxysilane, tetrapropoxysilane, as well as higher and mixed examples, i.e. silanes with longer alkyl chains or silanes with various alkyl residues.
Mixtures of various alkyl- or alkoxysilanes may also be used advantageously.
It may be necessary to prehydrolyse these compounds. The use of such siloxanes as need no pretreatment, such as e.g. polysiloxanes, is advantageous for the production of the pastes according to the invention.
When the substrate is heated in the diffusion furnace, the organic silicon compounds form silicon dioxide, e.g. according to the following equation:
C8H20O4Sixe2x86x92CO2+H2O+SiO2
(TEOS, tetraethyl orthosilicate)
In order to produce the dopant matrix, compounds of boron and phosphorus, as well as aluminium compounds, may be used as the inorganic components. Examples include boron oxide B2O3, boric acid B(OH)3, phosphoric acid H3PO4, P2O5 and Al salts.
Organic compounds of boron and phosphorus as well as organic aluminium compounds can be used as the organic components.
Additives may furthermore be mixed with the pastes according to the invention. These may be both organic and inorganic acids, such as e.g. nitric acid, hydrochloric acid, lactic acid, oxalic acid, formic acid or the like. In particular, these acids are added when hydrolysis of the organic Si compound needs to be carried out. Generally, such organic acids whose alkyl residues have a chain length of n=1-10 are usable. It is possible to use organic acids that have a residue R=OH or =alkyl with n=1-10. The alkyl residues of the salts that can be used may be both straight-chained and branched.
Corresponding additives in the pastes according to the invention are necessary when the organic silicon compound needs to be prehydrolysed. However, in the case of siloxane for example, this additive may be omitted.
Suitable thickening agents include Thixoton (ricinus oil), Borchigen TS (Theological additive for silicic acids), bentonites with various qualities and properties, generally inorganic rheology additives for various polar solvent mixtures, nitrocellulose, ethylcellulose and other cellulose compounds, polyvinylpyrrolidones with variable quality, starch (+NH4OH), gelatine, alginic acid, synthetic Mgxe2x80x94Naxe2x80x94Li silicate (Laponite), highly disperse amorphous silicic acid (Aerosil), polyvinylbutyral (Mowital) (highly soluble e.g. in butyl glycol), sodium carboxymethylcellulose (vivistar), thermoplastic polyamide resin (Eurelon) (needs to be boiled down at about 180xc2x0 C.), organic ricinus oil derivative (Thixin R), diamide wax (Thixatrol plus), swelling polyacrylates (Rheolate), polyetherurea-polyurethane (Rheolate), polyether-polyols (Rheolate) or Mgxe2x80x94Al hydrosilicate.
Particularly highly suitable additives are those that bind well with the other components, advantageously can be predried tack-free at temperatures below 200xc2x0 C. and burn without leaving residues at 200-500xc2x0 C.
The viscosity of the highly pure dopant paste is adjusted by the thickening agent, as well as by the solvent used.
A basic requirement for a good dopant paste in photovoltaic technology is the lowest possible contamination with metallic compounds. In particular, the cations of the metals iron, chromium, copper, nickel, gold, calcium etc., often referred to in the literature as carrier lifetime xe2x80x9ckillersxe2x80x9d, in particular, are undesirable. Their concentrations should be as far as possible below 200 ppb in the paste. In particular cases, the concentrations should as far as possible not exceed 100 ppb. In order to make it possible to comply with these conditions, in particular cases only purified starting materials can be used.
Besides the components mentioned so far, the addition of solvents is necessary for the production of the pastes.
Suitable organic solvents include hydrophilic polyalcohols (ethylene glycol, ethylene glycol monobutyl ether, ethylene glycol monophenyl ether, methoxymethoxyethanol, ethylene glycol monoacetate, ethylene glycol diacetate, diethylene glycol, diethylene glycol monomethyl ether, diethylene glycol monoethyl ether acetate, diethylene glecol monobutyl ether, diethylene glycol monobutyl ether acetate, diethylene glycol dimethyl ether, diethylene glycol methylethyl ether, diethylene glycol diethyl ether, diethylene glycol acetate, triethylglycol, triethylene glycol monomethyl ether, triethylene glycol monoethyl ether, tetraethylene glycol, liq. polyethylene glycols, propylene glycol, propylene glycol monomethyl ether, propylene glycol monoethyl ether, propylene glycol monobutyl ether, 1-butoxyethoxypropanol, dipropylglycol, dipropylene glycol monomethyl ether, dipropylene glycol monoethyl ether, tripropylene glycol monomethyl ether, polypropylene glycols, trimethylene glycol, butanedial, 1,5-pentanedial, hexylene glycol, oxylene glycol, oxylene glycol, glycerine, glyceryl acetate, glyceryl diacetate, glyceryl triacetate, trimethylolpropyne, 1,2,6-haxanetriol) or hydrophilic derivatives thereof.
Hydrophilic ethers (dioxane, trioxane, tetrahydrofuran, tetrahydropyran, methylal), diethylacetal, methyl ethyl ketone, methyl isobutyl ketone, diethyl ketone, acetonylacetone, diacetone alcohol or hydrophilic esters (methyl formate, ethyl formate, propyl formate, methyl acetate, ethyl acetate) may furthermore be used as solvents.
Tests have shown that application of a 50-100 mg quantity of paste to a customarily used wafer is sufficient to achieve a dopant effect. The application of the paste according to the invention to the wafer to be treated can be carried out with the methods known to the person skilled in the art. In particular, application by the screen-printing process is suitable for this purpose. The conversion of the applied paste into the desired dopant layers takes place, in the known fashion, by heating to temperatures of 200 to 600xc2x0 C., preferably to about 500xc2x0 C. The required heating time depends on the thickness of the layer of paste applied.
The paste according to the invention is suitable for use in chip production but, in particular, also for use in the production of solar cells used in solar technology.
For example, it has been found that through using boron pastes according to the invention for the production of doped glass layers, the efficiency of solar cells, which is usually 14% in production, is increased by about 1.5 to 2%.
For better understanding and for clarification, examples will be given below which fall within the scope of protection of the present invention but do not restrict the invention to these examples.